1. Field of the Invention
The present invention relates to a device and method for magnetically controlling the flow of a fluid along a wall and, more particularly, to magnetic control of the boundary layer on aerodynamic bodies (such as wings, rotors and flaps) and hydrodynamic bodies (such as submarine sails, bow-planes, stern appendages and propellers).
2. Description of the Prior Art
A viscous fluid, and a body completely immersed in the fluid, form a boundary layer at the body's surface when the fluid and the body move relative to each other. That is, the layer of fluid in contact with the body is essentially at rest, while in an area removed from the body, the fluid is moving at its free-stream velocity. The region between the body and that area is known as a boundary layer. PG,3
The boundary layer is laminar at low Reynolds' number. (Re=UL/.nu., where U is a characteristic velocity, such as the free-stream velocity, L is a characteristic dimension of the body, such as the length of a wing chord or boat hull, and .nu. is the kinematic viscosity of the fluid.) When the Reynolds' number increases, the boundary layer becomes unstable and turbulent. In some cases, it can "separate" from the body.
FIGS. 1(a) and 1(b) illustrate flow over an airfoil. It will be appreciated that the same principles apply whether the fluid is a liquid or a gas and regardless of the shape of the body.
When the airfoil 10 is operating at a small angle of attack .alpha., as shown in FIG. 1(a), the fluid stream 12, with a free-stream velocity U.sub.28, flows smoothly over the upper surface 14 of the airfoil. The downward deflection of the fluid stream by the airfoil causes an equal and opposite upward lift force to act on the airfoil.
As the angle of attack .alpha. increases, as shown in FIG. 1(b), the boundary layer may become turbulent, as indicated by the irregular flow 17. (For purposes of illustration, the boundary layer is depicted in FIG. 1 as much thicker than it is in actuality.) At very high angles of attack the boundary layer may separate from the airfoil, which then stalls. In addition to the loss of lift caused by boundary layer separation, eddies and turbulence 18 develop in the boundary layer.
Instability leading to boundary layer turbulence at a body's surface has several implications.
First, boundary layer turbulence increases viscous drag, which may create the need for additional propulsive force to be applied to the airfoil or other body, which in turn requires more fuel to be expended to maintain the speed of the airplane, submarine, propeller blade, etc., to which the airfoil is attached. Moreover, if the flow separates completely, additional pressure drag is created.
In addition, a turbulent boundary layer exhibits large velocity and pressure fluctuations, which induce noise. Noise can be a significant problem in many environments, one example being submarine control surfaces and propeller screw blades. Pressure fluctuations associated with boundary layer separation can cause vibration, which in turn causes fatigue, which can be a serious problem particularly in metal aircraft parts.
Various approaches have been taken to stabilize boundary layer flow and delay boundary layer separation. One of these approaches includes optimizing the geometry of the airfoil to achieve a maximum possible angle of attack. However, an optimum airfoil shape still only allows the airfoil to operate at limited angles of attack.
Approaches for controlling the boundary layer along a surface of an object have also included providing suction or injection of air through fine slits in the airfoil surface to supply or withdraw energy from the boundary layer. However, in addition to the burden of providing fine slits over the surface of the object, such approaches require extensive tubing networks to supply the force necessary for suction or injection. Accordingly, this approach adds considerably to the overall weight and complexity of the object, which is generally inconsistent with the design objectives of most applications.
The prior art has not yet achieved the capability to provide all of these types of boundary layer control in a very efficient, practical and easily implemented fashion.